lncRNA read-through regulates the BX-C insulator Fub-1

Epigenetic Regulation of Chromatin Functions by MicroRNAs and Long Noncoding RNAs and Implications in Human Diseases

The bulk of RNA produced from the genome of complex organisms consists of a very large number of transcripts lacking protein translational potential and collectively known as noncoding RNAs (ncRNAs). Initially thought to be mere products of spurious transcriptional noise, ncRNAs are now universally recognized as pivotal players in cell regulatory networks across a broad spectrum of biological processes. Owing to their critical regulatory roles, ncRNA dysfunction is closely associated with the etiopathogenesis of various human malignancies, including cancer. As such, ncRNAs represent valuable diagnostic biomarkers as well as potential targets for innovative therapeutic intervention. In this review, we focus on microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), the two most extensively studied classes in the field of ncRNA biology. After outlining key concepts of miRNA and lncRNA biogenesis pathways, we examine their multiple roles in mediating epigenetic regulation of gene expression and chromatin organization. Finally, by providing numerous examples of specific miRNAs and lncRNAs, we discuss how dysregulation of these mechanisms contributes to the onset and/or progression of various human diseases.

Molecular and genetic characterization of Cbx-Basel , a new dominant allele of Ultrabithorax in D. melanogaster

Dominant gain-of-function alleles for the homeotic gene Ultrabithorax ( Ubx ) have been known for a long time. They are summarized under the name Contrabithorax ( Cbx ). Such alleles are rather easy to spot because the morphology of the conspicuous dorsal wing appendage is often dramatically changed. The majority of these alleles is associated with chromosomal rearrangements that alter the genetic landscape of the Ultrabithorax locus. Thereby, UBX protein is ectopically expressed in the wing primordium where it is normally absent. Since Ubx specifies haltere identity, wing cells expressing UBX are determined to become haltere cells. However, apart from the prototypic allele Cbx-1 , information on the molecular details of Contrabithorax alleles is scarce. Here, we present a rather detailed account on a novel Cbx-1-like allele called Cbx-Basel . The results of our study corroborate the model that has been postulated for the Cbx-1 wing phenotype.

Dynamic modulation of enhancer-promoter and promoter-promoter connectivity in gene regulation

Enhancers are short segments of regulatory DNA that control when and in which cell-type genes should be turned on in response to a variety of extrinsic and intrinsic signals. At the molecular level, enhancers serve as a genomic scaffold that recruits sequence-specific transcription factors and co-activators to facilitate transcription from linked promoters. However, it remains largely unclear how enhancers communicate with appropriate target promoters in the context of higher-order genome topology. In this review, we discuss recent progress in our understanding of the functional interplay between enhancers, genome topology, and the molecular properties of transcription machineries in gene regulation. We suggest that the activities of transcription hubs are highly regulated through the dynamic rearrangement of enhancer-promoter and promoter-promoter connectivity during animal development.

Stem-loop and circle-loop TADs generated by directional pairing of boundary elements have distinct physical and regulatory properties

The chromosomes in multicellular eukaryotes are organized into a series of topologically independent loops called TADs. In flies, TADs are formed by physical interactions between neighboring boundaries. Fly boundaries exhibit distinct partner preferences, and pairing interactions between boundaries are typically orientation-dependent. Pairing can be head-to-tail or head-to-head. The former generates a stem-loop TAD, while the latter gives a circle-loop TAD. The TAD that encompasses the Drosophila even skipped (eve) gene is formed by the head-to-tail pairing of the nhomie and homie boundaries. To explore the relationship between loop topology and the physical and regulatory landscape, we flanked the nhomie boundary region with two attP sites. The attP sites were then used to generate four boundary replacements: λ DNA, nhomie forward (WT orientation), nhomie reverse (opposite of WT orientation), and homie forward (same orientation as WT homie). The nhomie forward replacement restores the WT physical and regulatory landscape: in MicroC experiments, the eve TAD is a 'volcano' triangle topped by a plume, and the eve gene and its regulatory elements are sequestered from interactions with neighbors. The λ DNA replacement lacks boundary function: the endpoint of the 'new' eve TAD on the nhomie side is ill-defined, and eve stripe enhancers activate a nearby gene, eIF3j. While nhomie reverse and homie forward restore the eve TAD, the topology is a circle-loop, and this changes the local physical and regulatory landscape. In MicroC experiments, the eve TAD interacts with its neighbors, and the plume at the top of the eve triangle peak is converted to a pair of 'clouds' of contacts with the next-door TADs. Consistent with the loss of isolation afforded by the stem-loop topology, the eve enhancers weakly activate genes in the neighboring TADs. Conversely, eve function is partially disrupted.

A Synergistic, Cultivator Model of De Novo Gene Origination

The origin and fixation of evolutionarily young genes is a fundamental question in evolutionary biology. However, understanding the origins of newly evolved genes arising de novo from noncoding genomic sequences is challenging. This is partly due to the low likelihood that several neutral or nearly neutral mutations fix prior to the appearance of an important novel molecular function. This issue is particularly exacerbated in large effective population sizes where the effect of drift is small. To address this problem, we propose a regulation-focused, cultivator model for de novo gene evolution. This cultivator-focused model posits that each step in a novel variant's evolutionary trajectory is driven by well-defined, selectively advantageous functions for the cultivator genes, rather than solely by the de novo genes, emphasizing the critical role of genome organization in the evolution of new genes.

Cis-regulatory modes of Ultrabithorax inactivation in butterfly forewings

Hox gene clusters encode transcription factors that drive regional specialization during animal development: for example the Hox factor Ubx is expressed in the insect metathoracic (T3) wing appendages and differentiates them from T2 mesothoracic identities. Hox transcriptional regulation requires silencing activities that prevent spurious activation and regulatory crosstalks in the wrong tissues, but this has seldom been studied in insects other than Drosophila, which shows a derived Hox dislocation into two genomic clusters that disjoined Antennapedia (Antp) and Ultrabithorax (Ubx). Here, we investigated how Ubx is restricted to the hindwing in butterflies, amidst a contiguous Hox cluster. By analysing Hi-C and ATAC-seq data in the butterfly Junonia coenia, we show that a Topologically Associated Domain (TAD) maintains a hindwing-enriched profile of chromatin opening around Ubx. This TAD is bordered by a Boundary Element (BE) that separates it from a region of joined wing activity around the Antp locus. CRISPR mutational perturbation of this BE releases ectopic Ubx expression in forewings, inducing homeotic clones with hindwing identities. Further mutational interrogation of two non-coding RNA encoding regions and one putative cis-regulatory module within the Ubx TAD cause rare homeotic transformations in both directions, indicating the presence of both activating and repressing chromatin features. We also describe a series of spontaneous forewing homeotic phenotypes obtained in Heliconius butterflies, and discuss their possible mutational basis. By leveraging the extensive wing specialization found in butterflies, our initial exploration of Ubx regulation demonstrates the existence of silencing and insulating sequences that prevent its spurious expression in forewings.

Blocking the blocker

An unexpected interaction between a long non-coding RNA locus and a genetic insulator called Fub-1 has an important role in gene regulation during development in Drosophila.